
EP 0409549 discloses a process for the chlorination of a sucrose-6-acylate in a tertiary amide reaction vehicle to produce a sucralose-6-acylate, such as sucralose-6-acetate. A large excess of an acid chloride, such as phosgene, is used as the chlorination agent in this process. Following the chlorination reaction, the excess chlorination agent is quenched using a suitable base, thereby forming the chloride salt of the base. The resulting product stream thus comprises a sucralose-6-acylate, the tertiary amide reaction vehicle, water, and salts.
A known method for obtaining sucralose from a product stream comprising a sucralose-6-acylate, a tertiary amide reaction vehicle, water, and salts, without isolation of the sucralose-6-acylate intermediate, is disclosed in EP 0708110. The process comprises deacylation of the sucralose-6-acylate before or after removal of the tertiary amide reaction vehicle, and then isolation of the sucralose. The removal of the tertiary amide (which is usually DMF) is carried out by steam stripping.
According to EP 0708110, it is preferred to perform the deacylation after the removal of the reaction vehicle, because otherwise, during the deacylation step, base-catalysed decomposition of the reaction vehicle, in this case a tertiary amide, occurs. This hinders the subsequent isolation of the sucralose, and also means that the tertiary amide cannot be efficiently recovered and recycled. Thus, the tertiary amide reaction vehicle is removed from an aqueous solution of sucralose-6-acylate, and deacylation of the sucralose-6-acylate is carried out thereafter.
The preferred process according to EP 0708110 requires that a large amount of water is present in the process stream during the removal of the tertiary amide. This is necessary to ensure that the high concentration of salts is maintained in solution, thereby minimising the amount of solids that the process stream has to accommodate. The large amount of water also ensures that the sucralose-6-acylate, which is soluble in the tertiary amide reaction vehicle but less soluble in water, is maintained in solution as the tertiary amide reaction vehicle is removed.
The large amount of water that is present in the process stream of EP 0708110 during the removal of the tertiary amide reaction vehicle has the effect that the removal of the tertiary amide reaction vehicle, for example by steam stripping, is very energy intensive. It would be advantageous if the amount of water present during this operation could be reduced, in order to increase the energy efficiency of the process.
The disadvantages of the known steam stripping process for removal of the reaction vehicle are discussed in WO 2005/090376 and WO 2005/090374. Here it is proposed to remove all liquids from the chlorination feed to provide a solid residue, and to then obtain sucralose from the solid residue. According to this prior art, the removal of the liquids preferably takes place using an agitated thin film dryer.
A further disadvantage associated with the process of EP 0708110 is that the salts are maintained in the process stream and are only removed during the final isolation of sucralose. The presence of high concentrations of salts in the process stream limits opportunities to employ salt-sensitive purification techniques on the process stream. Furthermore, removal of salts only during the final isolation of sucralose results in a waste stream that contains a high concentration of salts, as well as other impurities. The high level of salts in the waste stream makes the treatment of the waste stream difficult. It would be advantageous if the concentration of salts both in the process stream and also in the waste stream obtained during the final isolation of sucralose could be minimised.